1. Field of the Invention
This invention relates geophysical exploration for hydrocarbon and other valuable materials and related data processing.
2. Description of the Related Art
Geophysical exploration or seismic exploration in particular involves surveying subterranean geological formations for hydrocarbon deposits or other valuable materials. A survey typically involves deploying source(s) and receivers at predetermined locations. The sources generate waves, which propagate into the geological formations and are changed by their interaction with different geological formations. Part of the waves emitted by the sources reaches the receivers. In response, the receivers generate electrical signals to produce survey data. Analysis of the survey data can then indicate the characteristics of the subterranean geological formations, such as the presence or absence of probable locations of hydrocarbon deposits.
Depending on the location where a survey takes place, there are surveys in sea, on land or in transition zones. Marine seismic surveying is a method for determining the structure of subterranean formations underlying bodies of water. Marine seismic surveying may typically utilize seismic energy sources and seismic receivers located in the water which may be either towed behind a vessel or positioned on the water bottom from a vessel. The energy source may typically be an explosive device or compressed air system which generates seismic energy, which then propagates as seismic waves through the body of water and into the earth formations below the bottom of the water. As the seismic waves strike interfaces between subterranean formations, a portion of the seismic waves may reflect back through the earth and water to the seismic receivers, to be detected, transmitted, and recorded. The seismic receivers typically used in marine seismic surveying may be pressure sensors, such as hydrophones. Additionally, motion sensors, such as accelerometers, may be used. Both the sources and receivers may be strategically repositioned to cover the survey area.
Land seismic surveying is done on land. The energy sources are typically vibratory sources (vibrators). The vibrators produce a pressure signal that propagates through the earth into the various subsurface layers. Here elastic waves are formed through interaction with the geologic structure in the subsurface layers. Elastic waves are characterized by a change in local stress in the subsurface layers and a particle displacement, which is essentially in the same plane as the wavefront. Acoustic and elastic waves are also known as pressure and shear waves. Acoustic and elastic waves are collectively referred to as the seismic wavefield.